3 ms·

In principle, it seems you could handle the heterogenous case by designing an `Any` struct and (at least in C++) implicit constructors to take various types (in

by cfallin 11y ago

In principle, it seems you could handle the heterogenous case by designing an `Any` struct and (at least in C++) implicit constructors to take various types (int, T*, ...). At some point this starts to feel like designing a custom ABI and the complexity may not be worth it, but it seems there should be a way :-)

haberman 11y ago
> it seems you could handle the heterogenous case by designing an `Any` struct and (at least in C++) implicit constructors to take various types (int, T*, ...) In C11 you could also use _Generic. It will be perfect for the year 2045 when Microsoft adds C11 support.
- ryanprichard 11y ago
  _Generic might not work so well to handle int vs double vs pointer-to-T. Apparently, even unselected associations must type-check. e.g.: Here's a simple implementation: enum AnyKind { AnyInt, AnyDouble, AnyStr }; struct Any { enum AnyKind kind; union { int i; double d; const char *s; } u; }; #define ANY(x) \ _Generic((x), \ int: (struct Any){ .kind = AnyInt, .u.i = (x) }, \ double: (struct Any){ .kind = AnyDouble, .u.d = (x) }, \ const char *: (struct Any){ .kind = AnyStr, .u.s = (x) }) int main() { ANY(1); ANY(1.0); ANY("abc"); } Clang gives errors like: error: initializing 'const char *' with an expression of incompatible type 'double' This issue was discussed a bit on the GCC bugtracker: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64509 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64509 Qualifiers might also be an issue. My impression so far is that every combination of differently qualified type is supposed to require a different _Generic association. (GCC 5.2 and Clang 3.7 differ here.) That would require a huge number of _Generic associations, but it might be possible to strip the qualifiers using a comma operator, _Generic( (0,(x)), ... ). A comma expression is not an lvalue, so perhaps it cannot be qualified. DR423[1] askes that question. [1] http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_423.htm http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_423.htm
  - haberman 11y ago
    What a fantastically informative reply, thanks for this! Having not used _Generic myself, it's good to know what kind of limitations to expect.

by cfallin 11y ago

haberman 11y ago

> it seems you could handle the heterogenous case by designing an `Any` struct and (at least in C++) implicit constructors to take various types (int, T*, ...) In C11 you could also use _Generic. It will be perfect for the year 2045 when Microsoft adds C11 support.

ryanprichard 11y ago
_Generic might not work so well to handle int vs double vs pointer-to-T. Apparently, even unselected associations must type-check. e.g.: Here's a simple implementation: enum AnyKind { AnyInt, AnyDouble, AnyStr }; struct Any { enum AnyKind kind; union { int i; double d; const char *s; } u; }; #define ANY(x) \ _Generic((x), \ int: (struct Any){ .kind = AnyInt, .u.i = (x) }, \ double: (struct Any){ .kind = AnyDouble, .u.d = (x) }, \ const char *: (struct Any){ .kind = AnyStr, .u.s = (x) }) int main() { ANY(1); ANY(1.0); ANY("abc"); } Clang gives errors like: error: initializing 'const char *' with an expression of incompatible type 'double' This issue was discussed a bit on the GCC bugtracker: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64509 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64509 Qualifiers might also be an issue. My impression so far is that every combination of differently qualified type is supposed to require a different _Generic association. (GCC 5.2 and Clang 3.7 differ here.) That would require a huge number of _Generic associations, but it might be possible to strip the qualifiers using a comma operator, _Generic( (0,(x)), ... ). A comma expression is not an lvalue, so perhaps it cannot be qualified. DR423[1] askes that question. [1] http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_423.htm http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_423.htm
- haberman 11y ago
  What a fantastically informative reply, thanks for this! Having not used _Generic myself, it's good to know what kind of limitations to expect.